Radio-frequency filter



2 Sheet'sSheet 1 Filed Dec. 27, 1946 F/Gl.

406/! FREQUENCY TRAN-9M! TTEI? BROADCAST TRANSMITTER rC/ H //v l/EN TORRH 5714/ TH ATTORNEY July 11, 1950 P. H. SMITH RADIO FREQUENCY FILTER 2Sheets-Sheet 2 Filed Dec. 27, 1946 I lNl/E/VTOR R H. SM/ TH H III; II

ATTORNEY Patented July 11, 1950 RADIO-FREQUENCY FILTER Phillip H. Smith,Fair Haven,-N. .I., assignor to Bell Telephone Laboratories,Incorporated, New York, N. Y., a corporation of New York ApplicationDecember 27, 1946, Serial No. 718,642

9 Claims.

This invention relates to wave transmission and more particularly towave filters.

An object of the invention is to couple a grounded coaxial transmissionline to I an ungrounded coaxial transmission line.

Another object is to transmit freely a desired band of frequencies fromone to the other of such coupled lines while suppressing undesiredfrequencies.

A further object is to prevent the escape of energy when two suchtransmission lines are coupled together.

In high frequency transmission systems it is sometimes required toconnect a grounded coaxial transmission line to an ungroundedtransducer, which may be another coaxial line. For example, a coaxialline may be used to connect a high frequency transmitter to an antennamounted on top of a series-fed, base-insulated standard broadcast towerradiator. As the outer conductor of this line is normally grounded atransmission network of special type must be inserted in the line at thepoint Where it crosses the base insulator, to prevent grounding thetower and thereby disabling its use as a vertical antenna. Such atower-base bridging filter must appear as practically an open circuit inthe standard broadcast range but must freely transmit, withoutradiation, a sufliciently wide band of frequencies in the high frequencyrange.

In accordance with one embodiment of the invention a filter suitable forthis purpose comprises a section of coaxial transmission line, twocapacitors connected in series with the inner conductor and spacedpreferably less than a half wavelength at a frequency to be passed, acircumferential gap in the outer conductor, and a quarter-wave coaxialskirt connected at one end to one side of and encircling the gap. At itsinner end the skirt presents a very low impedance at the frequency to bepassed. When the capacitors have the proper values the filter will passa sufficiently wide high frequency band and substantially match the lineimpedances at each end. The capacitors are preferably made variable andeach may comprise a coaxial rod inserted into the end of the hollowinner conductor of the coaxial section. To prevent the escape of energyat the open end of the skirt a second similar skirt may be connected tothe connected end of the first skirt so as to encircle it. This secondskirt may be made adjustable axially for precise tuning. However, thefilter is substantially an open circuit at the standard broadcastfrequencies which may be associated with the tower and the ungroundedportion of the line. To provide additional suppression for the energy inthis latter range, a short-circuited quarterwave stub line may beshunted across one end of the filter.

The nature of the invention will be more'fully understood from thefollowing detailed descrip-v tion and by reference to the accompanyingdrawing, in which like referencecharacters refer to similar orcorresponding parts and in which:

Fig. 1 shows an antenna system employing a tower-base bridging filter inaccordance with the invention;

Fig. 2 is a vertical sectional view, partly schematic, of one embodimentof the filter;

. Fig. 3 shows the equivalent filter circuit at a transmitted frequency;Fig.4 is the equivalent circuit at a suppressed frequency; and I Fig. -5is a vertical sectional view of the filter showing certain of thefeaurtes in greater detail. In Fig. 1 a tower I of conventional designmounted at its base on insulators 2 provides a vertical antennainsulated from ground. The tower I is series-fed from a broadcasttransmitter 3 one output lead 4 of which is connected near the base andthe other 5 of which is grounded.

On top of the tower I is mounted a high frequency antenna 6 which is fedfrom a high frequency transmitter I through a coaxial transmission line.The portion 8 of the transmission line which runs up the tower I must beinsulated from ground so that the tower will function properly as avertical radiator for the transmitter 3. However, the portion 9 of thecoaxial line extending from the tower I to the transmitter I willordinarily have its outer conductor grounded, as shown at Ill.Therefore, in accordance with the invention, a tower-base bridgingfilter II of special design is inserted between the grounded portion 9and the ungrounded portion 83 of the coaxial feed line.

The transmittert may, for example, be of the amplitude modulation typehaving a frequency fl falling within the standard broadcast range of 550to 16 00 kilocycles while the transmitter I employs frequency modulationat a frequency f2 within the range from 88 to I08 megacycles.

As shown somewhat schematically in Fig. 2, one embodiment of the filterI I comprises a section of coaxial transmission line I2 having twocapacitors CI and C2, preferably variable, connected in series with itsinner conductor I3. The capacitors CI and C2 are spaced apart a distanceD preferably less than half a wavelength at the frequency f2 to betransmitted. The

outer conductor I4 of the coaxial section I2 has a circumferential gapI5, preferably opposite the capacitor CI. A conductive coaxial skirt I6having a length approximately equal to a quarter wavelength at thefrequency f2 encircles the gap I5 and is connected at one end to a sidethereof through the conductive annular member I1, but is left open atthe opposite end I8. A

second similar coaxial skirt l9 of'approximately the same length but oflarger diameter is connected to the connected end of the first skirt I6,as shown at 20, so as to encircle the first. skirt. The outer skirt I9is preferably made so that it can be moved axially along the skirt l6for adjustment of the efiective length of the coaxial line section 2|formed therebetween. A section of coaxial line 22 short-circuited at itsouter end 23 and having a length approximately equal to a quarterwavelength at the frequency 2 is connected in shunt at one end of theline section I2, preferably near the capacitor C2 at the end connectedto the grounded portion 9 of the feed line.

The operation of the filter H will now be explained in connection withthe circuits of Figs. 3 and 4. As shown in Fig. 3 the equivalentelectrical circuit at the transmitted frequency f2 consi'sts only of thesection of line I2 of length D with the series capacitors CI and C2 atits ends. The outer conductor I4 and the sleeve IE cons'titute an openquarter-wave line section 24 the impedance of which viewed at the gap I5is substantially zero. The short-circuited quarterwave line section 2|is adjusted to provide a substantially infinite series impedance at theopen end of the line section 24 and thus prevent the escape of anyenergy of the frequency f2 flowing in the section I2. As viewed at thepoint of con: nection the stub section 22 has a substantially infiniteimpedance at the frequency f2 but a low impedance at the frequency fl,and therefore effectively prevents the passage of energy from the.broadcast transmitter 3 through the filter II to the high frequencytransmitter I.

The proper adjustment of the capacitors Cl and 02 will now beconsidered. A series reactance, such as CI, at one point in a uniform,dissipationless transmission line may be adjusted toa value such thatthe impedance looking into the line at a second point, located a fixeddistance D from the first point, is conjugate to the impedance at thefirst point. Connecting at the second point a second series reactance,such as C2, of the same character and magnitude as the first will makethe impedance of the combination just equal to the characteristicimpedance of the line. That is, the reflection caused by the firstreactance is completely cancelled out by the second reactance. lengthwithin wide limits, but to minimize the overall length of the filter, itis preferably made less than a. half wave-length at the frequency 12 tobe passed.

When the capacitors CI and C2, are adjusted in the manner explainedabove, the circuit of Fig. 3 will be. efiectively equivalent to a smoothsection of transmission line, that is, a low-loss one to. onetransformer having an image impedance at each end equal to thecharacteristic impedance ofv the line section I2. The filter will,therefore, transmit a wide band of frequencies including f2. and willmatch in impedance the coaxial lines 8 and 9 connected to theendsthereof.

Fig. 4 shows the equivalent circuit for the filter II at the frequencyfI as viewed at the open end I8 of the line section 24, which is theonly point at which energy of this frequency may enter. There is onepath from the skirt It to the ground Ill through the capacitance C3,which represents the capacitance between the skirt I6 and the outerconductor I4. Since this capacitance is comparatively small itsreactance will be high. A second path to ground comprises the impedancesZI, CI, C2 and Z2 in series. ZI is the impedance looking from the filterII into the line section 8 and His the impedance looking into the linesection 9. The capacitors CI and C2 also have high reactances at thefrequency fl. It is seen, therefore, that the filter offers a highimpedance to ground for energy of the frequency fl which is beingradiated by the tower I. In practice this impedance can be made from 10to 100 times the impedance to ground across the insulators 2. Theinductance L in shunt withZE represents the stub line 22. It may have areanctance of an ohm or less at fl andthus assists greatly in preventingenergy of this frequency from reaching the transmitter I. i

Fig. 5 shows in greater detail the mechanical structure of an embodimentof the filter II. The skirts I6 and I9 and the stub line 22 aresubstantially the same as those shown in Fig. 2. The coaxial linesection I2, however, has a hollow inner conductor 25 spaced from theouterconductor I 4 by the insulating rings 26. Each of the" coaxiallines 8 and 9 connected to the ends of the filter II also has a hollowinner conductor 2?.

Each of the variable capacitors CI and C2 comprises a section ofopen-circuited coaxial line 28 constituted by the hollow inner conductor25 and a conductive rod 29 which extends into an end thereof. The otherend of the rod 29 extends into the hollow inner conductor 2'5 and isslidably mounted in a conductive chuck 30. The eifective length of theline section 28, and therefore the capacitance, is adjusted by insertingthe? endof' the rod 29 a greater or less distance into the conductor 25.The proper values forthe'f capacitors .CI and C2 are most easily foundby trial, but they ordinarily will be approximatelyequal. r A cylinder3| of insulating material attached at one end to the conductor M bymeans of the annular flanged member 32 and at the other end to thesleeve I6 serves to hold the parts of the filter II. together and alsoprotects the interior of the filter from the weather. The flanges 33facilitate the connection of other portions of the coaxial feed line.

The distance D may be any The section of line 24 maybe of fixed lengthand is preferably made a'quarter wavelengthlong' at the high frequencyend' of the, range over which the filter II 15130 be tunable. Then, atlower frequencies within the range the'impedance of the line 2 5, at thegap I 5, will always be capacitive and its reactance will be small inmagnitude. This capacitance is" in series with Cl and maybe consideredto be a part thereof when the gap i5 is opposite the capacitance CI, asshown. Therefore, at the lower frequencies CI maybe slightly increasedin value to compensate for the capacitance of the line 2%.

The stub line '22 may also be of fixed length and is preferably made aquarter wavelength long at the center of the tunable range offrequencies. Its reactance will change over the range but is almostexactly equivalent to a' small series reactance at the same positionalong the line. The effect of this equivalent series reactance may beexactly compensated by an adjustment of C2 provided the stub 22 isconnected near the capacitor C2, as shown. This compensation willrequire the value of C2 to be re-- duced at the upper end of the rangeand increased at the lower end.

It is evident that the filter II is a weatherproof, gas-tight,non-radiating structure which is relatively low in cost, small in size,light in weight and easy to install, having a sufliciently highimpedance at the frequency fl but low loss over a Wide band centered atthe frequency f2. Furthermore, for optimum performance at any specifiedfrequency f2 within a wide range only three elements, the rods 29 andthe sleeve l9, require adjustment, which is easily made with the aid ofa calibration curve and an ordinary ruler.

What is claimed is:

1. A wave filter for connecting a grounded coaxial transmission line toan ungrounded coaxial transmission line comprising a section of coaxialtransmission line, two capacitors connected in series with the innerconductor of said section with a spacing therebetween of less than ahalf wavelength at a frequency to be passed by said filter, acircumferential gap in the outer conductor of said section approximatelyopposite one of said capacitors, a conductive coaxial skirt connected atone end to one side of and encircling said gap, and a second similarskirt connected to the connected end of and encircling said first skirt,each of said skirts having a length approximately equal to a quarterwavelength at said frequency and said capacitors being adjusted toprovide for said filter image impedances which are substantially equaland each of which is substantially equal to the characteristic impedanceof said section of line.

2. A filter in accordance with claim 1 in which said capacitors arevariable.

3. A filter in accordance with claim 1111 which the inner conductor ofsaid section is hollow and one of said capacitors comprises a conductiverod extending for an adjustable distance into the end of said innerconductor.

4. A filter in accordance with claim 1 which includes a short-circuitedsection of coaxial transmission line connected in shunt at one end ofsaid filter and having a length approximately equal to a quarterwavelength at said frequency.

5. A filter in accordance with claim 1 which includes a short-circuitedsection of coaxial transmission line connected in shunt at one end ofsaid filter near the other of said capacitors and having a lengthapproximately equal to a quarter wavelength at said frequency.

6. A filter in accordance with claim 1 in which the inner conductor ofsaid section is hollow and, each of said capacitors comprises aconductive rod extending for an adjustable distance into an end of saidinner conductor.

7. A filter in accordance with claim 1 in which said capacitors areapproximately equal in capacitance.

8. A filter in accordance with claim 1 in which said second skirt isadjustable axially with respect to said first skirt.

9. A filter in accordance with claim 8 in which said capacitors arevariable.

PHILLIP H. SMITH.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

